1. Technical Field of the Invention
The present invention relates to laminated piezoelectric elements for use in, for example, piezoelectric actuators of fuel injectors.
2. Description of the Related Art
In recent years, laminated piezoelectric elements have come to be used in piezoelectric actuators for the purpose of obtaining large actuating motions with low voltages. The laminated piezoelectric elements are generally formed by integrally firing a plurality of thin piezoelectric ceramic layers that are alternately laminated with a plurality of internal electrode layers. The number of piezoelectric ceramic layers in a laminated piezoelectric element is generally 100-700, and the thickness of the piezoelectric ceramic layers is generally 20-200 μm.
With such an integrally-fired structure, internal stress induced in a laminated piezoelectric element in a direction hindering the actuating motion created by the element increases with the number of piezoelectric ceramic layers in the element. An excessive internal stress will cause cracks to occur in the laminated piezoelectric element, thereby deteriorating the characteristics of the element (e.g., decreasing the amount of the actuating motion) and lowering the reliability of the element (e.g., causing a short circuit).
To solve the above problem, there has been developed a method according to which: a plurality of unit laminates are first formed separately, and then stacked and bonded together with an adhesive to form a laminated piezoelectric element. As the adhesive, a heat-curing silicone resin adhesive which contains a platinum catalyst is generally used.
However, the inventors of the present invention have found, through experimentation, a problem with the above method. More specifically, a laminated piezoelectric element made by the above method causes a greater loss in the actuating motion created by the element than that having the integrally-fired structure. Further, when used in a fuel injector, this loss will cause a loss in the displacement of an injection valve, resulting in a failure in the fuel injection of the fuel injector.
To effectively suppress the loss in the actuating motion, the adhesive layers, each of which is formed of the silicone resin adhesive between adjacent two of the unit laminates, are required to have a small thickness of, for example, 1 μm or less.
However, with the decrease in the thickness of the adhesive layers, the amount of the platinum catalyst in each of the adhesive layers is accordingly decreased. Consequently, when anticatalysts, such as sulfur, amine, and phosphorus, adhere to and react with the adhesive layers, the curing of the adhesive layers is degraded, causing the adhesive strength of those layers to be decreased.
Further, due to the decreased adhesive strength, it is impossible to ensure high reliability of the laminated piezoelectric element when the element is subject to a high temperature, for example 120° C. or higher, during its operation in a fuel injector.